1. Field of the Invention
The present invention relates to transmission of an intensity-modulated light signal and, more particularly, to an apparatus for transmitting low-distortion, high-quality light signals over long distances.
2. Description of the Related Art
Optical transmission using optical fibers as transmission media has been known as a method especially adapted for long-distance transmission because of their low losses. Methods of converting electrical signals into light signals are roughly classified into direct modulation and external modulation. At present, the direct modulation method prevails because of its low cost. This direct modulation method chiefly uses a semiconductor laser as a light source. The electrical current injected into this laser is amplitude-modulated by an electrical signal which should be sent. In this way, the intensity of the output light is modulated and converted into a light signal.
As mentioned above, optical transmission is intrinsically suited for long-distance transmission because of low losses of optical fibers. However, where a light signal is transmitted by the direct modulation method, the wavelength of the light signal is distorted, especially in long-distance transmission. This severely limits the transmission distance. The main causes of distortions of light signals are described below.
One cause is that chirping, or fluctuations of the wavelength, of a semiconductor laser used as a light source is affected by the wavelength-dependence of the optical transmission characteristics of optical fiber paths and produces distortions. See M. R. Philips et al., "Nonlinear distortion from fiber dispersion of chirped intensity modulated signals", OFC'91,TuC4,p10,1991. As described above, the output light intensity is modulated by modulating the current injected into the semiconductor laser. Generally, a semiconductor laser is designed so that it oscillates at a single wavelength. However, if the injected current is varied, the oscillation wavelength is varied too. On the other hand, optical transmission paths consisting of optical fibers often have characteristics including transmission loss and transmission velocity which are affected by the light wavelength. Accordingly, if an intensity-modulated light signal involving wavelength used chirping as described above is transmitted by the use of an optical transmission path as mentioned above, light signal transmission loss and transmission velocity vary, depending on the intensity of the light signal, thus resulting in waveform distortions. Also, where an optical fiber amplifier is used, the gain of the amplifier has dependence on the wavelength and so if a light signal emitted from a semiconductor laser involving a large amount of chirping is amplified, then the waveform of the output light is distorted strongly.
A second cause is a distortion caused by interference of plural light signals which reach an opto-electronic converter device via their respective paths provided that reflection points exist in an optical transmission path. See J. H. Angenet et al., "Distortion of a multicarrier signal due to optical reflections.", ECOC91&IOOC, WeC8-4, pp569-571,1991. For example, as shown in FIG. 4, if plural reflection points R1 and R2 exist in an optical transmission path, one conceivable light signal incident on the light-receiving device of the opto-electronic converter device is light L1 coming directly from the light-emitting device of the electro-optic converter device. Another conceivable light signal incident on the light-receiving device is light L2 which reaches this receiving device after being reflected at the reflection point R2 and then at the reflection point R1. When two waves of light enter the light-receiving device in this way, if the correlation between the phases of the two waves is large, i.e., if the phase difference between the two waves is small, then interference occurs, producing a distortion.
The conceivable main causes of distortions in the prior art optical transmission apparatuses are the two factors described above. In order to prevent distortions due to the first factor, a semiconductor laser which produces a small amount of chirping when directly modulated should be used. Distortions attributed to the second factor can be effectively prevented by transmitting signals by means of light having a wide optical spectral line width because, if the optical spectral line width is wide, the probability that the two light signals L1 and L2 shown in FIG. 4 are the same in phase is low.
However, a semiconductor laser which is designed so that the amount of chirping is small has a narrow optical spectral line width because of its structure. Consequently, it is very difficult to develop a semiconductor laser which satisfies these two conditions simultaneously.
Generally, fiber-optic transmission paths used as main lines in an optical transmission-and-distribution network may be used to convey signals over long distances. Therefore, if the amount of chirping of transmitted light is large, that is, if the optical spectral line width is wide, waveform distortions caused by the wavelength dependence of transmission loss of the optical fiber paths and by the dispersion characteristics (i.e., the dependence of the transmission velocity on wavelength) produce greater effects. In contrast, optical fiber paths of a distribution system which branches off from a main line and is laid to each optical subscriber are comparatively short. Therefore, even if the amount of chirping is large ,that is, the optical spectral line width of the transmitted light is wide, waveform distortions due to the above-described transmission characteristics of the optical fibers are small.
Since high-performance components are not used as optical devices and optical terminals connected to the optical fiber transmission paths of the distribution system because of limitations imposed on the costs, the used optical components may be cheap ones which do not have a large amount of return losses. In this case, if the optical spectral line width of the transmitted light is wide, distortions due to multiple reflections of light as described in connection with FIG. 4 can be reduced to a minimum.
Because of the characteristics of the optical transmission paths described thus far, the optical spectral line width of transmitted light preferably has two conflicting characteristics, i.e., the line width is narrow in long-distance fiber-optic transmission paths of main lines and wide in low-quality optic-fiber transmission paths of distribution systems.